Photograph by Stephen Alvarez National Geographic August 2008
Rocky asteroids and icy comets have bombarded Earth since its formation 4.5 billion years ago. But perhaps the most famous impact of all was caused by the six-mile-wide comet or asteroid that annihilated the dinosaurs 65 million years ago. While impacts have been rare in our era, objects large enough to cause a planetwide disaster still pass ominously close to Earth's orbit, and the threat of collision remains.
Fortunately, we're learning how to protect ourselves from these cosmic invaders. New telescope systems with powerful cameras will aid in finding tens of thousands of small asteroids and comets large enough to destroy a city or state. Scientists are working on methods to deflect an incoming body using ingenious systems that over time can push or pull an object away from a collision course with Earth. Other scientists propose bombing large objects into small, less threatening pieces.
Although our planet has been bombarded for eons, only 174 scars from impacts have been found. Many hundreds or thousands of others have been obliterated by erosion or vegetation or hidden under the ocean. The impact craters are the result of tremendously destructive forces, yet many are beautiful. Geologists see them as a window into the early Earth, helping to answer questions about how it and other terrestrial planets formed and evolved.
Stone, Richard. Target Earth. National Geographic (August 2008), 134-49.
Lu, Edward T., and Stanley G. Love. "A Gravitational Tractor for Towing Asteroids." Nature (November 10, 2005), 177-78.
Asteroids and comets are relatively small objects—they're often called the leftover building blocks of the solar system. Though widely dispersed, they've had—and continue to have—a profound influence on the evolution of the planets, including Earth.
Asteroids can range from boulder- to moon-size (a few feet to hundreds of miles across) and are usually composed of rock, although some are a mix of nickel and iron. Usually irregular in shape, some are delicate rubble piles barely held together by their own gravity, while others are solid chunks of metal. The majority of asteroids orbit between Mars and Jupiter in a region called the asteroid belt; others are in orbits closer to the sun and can intersect with Earth's path. If the alignment is just right, Earth and asteroid will collide, sometimes with catastrophic results.
Comets travel much farther out in the solar system than asteroids do, either beyond the orbit of Neptune or in a huge, distant cloud surrounding the outer solar system called the Oort cloud. They're made of chunks of ice, dust, and rocky rubble and are usually a half mile to six miles across. Comets take anywhere from a few years to thousands of years to orbit the sun, but if they’re dislodged from their orbits by gravity and flung toward the sun, gases vaporizing off the surface create the familiar "tail" we see. A recent, gorgeous example was comet Hale-Bopp, which had such a bright tail it could be seen through the glare of city lights when it graced the sky in 1997. Beauty is one thing; however, a collision with Earth could mean disaster.
Our knowledge of asteroids and comets is growing dramatically as powerful ground-based telescopes detect increasingly more of them and spacecraft voyage to—and even land atop—them. Recent missions include NEAR, which landed a craft on the asteroid Eros in February 2001; Stardust, which brought comet samples back to Earth; and Deep Impact, which slammed a probe into comet Tempel 1 in July 2005.
Beatty, Kelly, and others. The New Solar System, 4th ed. Sky Publishing Corporation, 1999.
Gribbon, John. Companion to the Cosmos. Weidenfeld & Nicolson, 1996.
Mitton, Jacqueline. Cambridge Dictionary of Astronomy. Cambridge University Press, 2001.
Almost all of the objects that could collide with Earth are asteroids. By 2020 scientists hope to identify and catalog 90 percent of those whose size (140 meters or greater in diameter) and orbit (within 46 million miles of Earth's orbit) could result in a catastrophic impact. With sufficient warning, we might be able to avert danger. However, large comets can sneak up on us, and if one does, we might not have time to deflect a collision that could result in planetwide destruction.
With the ability to discover more of the objects that pass close to Earth comes a new set of terms to describe them:
Near-Earth Object (NEO): Comets and asteroids that come within 121 million miles of the sun.
Potentially Hazardous Object (PHO): A comet or asteroid at least 150 meters (460 feet) across that comes within 4.6 million miles of Earth’s orbit.
Potentially Hazardous Asteroid (PHA): A subset of PHOs—99 percent of PHOs are asteroids. A PHA that comes close to Earth's orbit won't necessarily collide with Earth. As of June 2008, there were 957 known PHAs, none of which was on a collision course with Earth.
Potentially Hazardous Comets (PHC): Comets are a small percentage of the PHO hazard; nonetheless even a large one could be on an Earth-crossing trajectory and remain unseen until it was too close to deflect.
Near-Earth Object Program.
Collins, Gareth S., H. Jay Melosh, and Robert A. Marcus. "Earth Impact Effects Program." Meteoritics & Planetary Science, Vol. 40, no. 6 (2005), 817–40.
There are 174 impact craters of varying ages and sizes discernible on Earth's surface. An unknown number of objects have vaporized before impact or missed Earth by a relatively short distance. Among the notable impacts and near misses are:
Manicouagan crater, Quebec: 60 miles wide, created 210 million years ago by an object perhaps four miles in diameter.
Chicxulub crater, Yucatán Peninsula: 110 miles wide, created 65 million years ago by a six-mile-diameter object.
Meteor Crater (Barringer Crater), Arizona: 0.75 miles wide, created 50,000 years ago by an object 150-300 feet in diameter.
Tunguska, Siberia: Meteorite exploded before impact, leveling 800 square miles of forest in 1908.
Most recent: Near Lake Titicaca, Peru, a 50-foot-wide object hit in September 2007.
Near miss: A 200-foot-diameter asteroid, 2002 MN, came within 74,000 miles of Earth on June 14, 2002.
Exploring Space: The Universe in Pictures. National Geographic, 2004.
Kerr, Richard. "Too Much Splat." Science (March 28, 2008), 1757.
Interactive map of craters worldwide.
A telling of the Aborignal legend of the formation of Gosses Bluff in Australia.
Our nearby celestial neighbors—the moon, Mercury, Venus, and Mars—all have pockmarked faces from eons of bombardment by asteroids and comets. With limited tectonic or erosional forces to erase them, the scars remain visible on the surface, offering insight into each body's evolution. A stunning example on Mars, now being explored by the long-lived Opportunity rover, is the half-mile-wide Victoria crater near the planet's equator, which was formed 10 million to 100 million years ago. Collisions have also occurred with the outer planets Jupiter, Saturn, Uranus, and Neptune, but we can't see the evidence because of the planets' gassy atmospheres (a rare exception is the succession of impacts on Jupiter in July 1994, which left huge, temporary bruises . Many of the outer planets' moons have impact craters that reveal a violent past. Missions to the planets by a legion of spacecraft and robotic rovers are supplying answers to basic geologic questions—and raising new ones.
Spencer, John R., and Jacqueline Mitton. The Great Comet Crash. Cambridge University Press, 1995.
Harrington, J., and others. "Lessons From Shoemaker-Levy 9 About Jupiter and Planetary Impacts." In Jupiter: Planet, Satellites & Magnetosphere, ed. F. Bagenal, T. E. Dowling, and W. McKinnon. Cambridge University Press, 2004.
Kerr, Richard. An Early Big Hit to Mars May Have Scarred the Planet for Life. Science (April 11, 2008), 165-66.
Other National Geographic Resources
Douthitt, Bill. "Beautiful Stranger, Saturn's Mysteries Come to Light." National Geographic (December 2006), 38-57.
Map supplement: "Eight Planets: The New Cosmic Order." National Geographic (December 2006).
Gore, Rick. "The Planets: Between Fire and Ice." National Geographic (January 1985), 4-51.
Handwerk, Brian. "Mars May Have Hosted Potentially Habitable Lake." National Geographic News, May 10, 2008.
Newcott, William R. "Venus Revealed." National Geographic (February 1993), 36-59.
Newcott, William R. "Return to Mars." National Geographic (August 1998), 2-29.
Newcott, William R. "In the Court of King Jupiter." National Geographic (September 1999), 126-139.
Petit, Charles W. "Making a Splash on Mars." National Geographic (July 2005), 58-77.
Sawyer, Kathy. "A Mars Never Dreamed Of." National Geographic (February 2001), 30-51.
Map supplement. "Mars Revealed: A New Look at Forces That Shape the Desert Planet." National Geographic (February 2001).
Exploring Space: The Universe in Pictures. National Geographic, 2004.
Last updated: July 1, 2008